1. Field of the Invention
The present invention relates to a circuit board, and more particularly, to a circuit board including a rigid region and a flexible region.
2. Description of the Related Art
As a conventional circuit board, for example, a printed wiring board described in Japanese Unexamined Patent Application Publication No. 2007-324208 is known. The printed wiring board will be described below with reference to the drawing. FIG. 14 is a cross-sectional structural view of a printed wiring board 500 described in Japanese Unexamined Patent Application Publication No. 2007-324208.
The printed wiring board 500 includes a first layer 510, a second layer 520, a flexible sheet 530, and conductor patterns 532a and 532b. The first layer 510 and the second layer 520 are stacked. The second layer 520 is partially cut out. The flexible sheet 530 is bonded to the first layer 510 in the cutout portion of the second layer 520. Thus, a region defined by the first layer 510 and the flexible sheet 530 (hereinafter referred to as a flexible region R503) becomes softer than regions where the first layer 510 and the second layer 520 are provided (hereinafter referred to as rigid regions R501 and R502). For this reason, the printed wiring board 500 can be bent at the flexible region R503.
Stress concentrates at boundaries between the rigid regions R501 and R502 and the flexible region R503 when the flexible region R503 is bent. More specifically, the hardness greatly changes at the boundaries between the rigid regions R501 and R502 and the flexible region R503. In this case, when the flexible region R503 is curved, portions of the flexible region R503 adjacent to the rigid regions R501 and R502 tend to curve with a small radius (that is, to be bent), and therefore, the portions may break. Accordingly, in the printed wiring board 500, the conductor patterns 532a and 532b are provided at the boundaries between the rigid regions R501 and R502 and the flexible region R503. This reinforces the boundaries between the rigid regions R501 and R502 and the flexible region R503.
The conductor patterns 532a and 532b are formed by metal films made of silver or copper, for example. While the conductor patterns 532a and 532b formed by the metal films have high rigidity, they plastically deform when bent with a small radius. The plastically deformed conductor patterns 532a and 532b no longer sufficiently function as reinforcing members. As a result, breakage may occur at the boundaries between the rigid regions R501 and R502 and the flexible region R503.